Inertial microfluidic systems have shown great potential for miniaturization of flow cytometry by removal of the sheath flow while maintaining high throughput (approximately 1 m/s). Force fields such as acoustic, electric, magnetic fields can also be used to manipulate particles within flow. However, efficiencies of these force field mediated methods degrade with increasing flow rate, which in turn limits the throughput (to approximately 0.1-1 mm/s). In inertial microfluidic systems, particles are laterally focused to a few distinct equilibrium positions and ordered with regular spacing, which is determined by the particle Reynolds number. There has been no report of control of particle spacing in flows.
Microscale particles in flow can be found in many fields of science and technology. One example is cells in the blood stream. Thus, control of particle motion/position in flow has numerous applications. “Local concentration” at microfluidic scale deviates from bulk concentration; with small sample volume standard deviation can be comparable to the mean of population. Most relevant application areas that can benefit from ordered particle streams include flow cytometry, cell printing, and metamaterial synthesis.
Inertial migration of particles in finite-Reynolds-number flow has been extensively studied experimentally and theoretically since the “tubular pinch” effect was experimentally first reported in 1961 by Segré and Silberberg. A rather unknown fact is that Segré and Silberberg also noted that particles tended to align in “necklaces” in the flow direction while remaining focused to an annulus. This dynamic self-assembly phenomenon was recently revisited in macroscale and microscale channel systems. Although a scaling of inter-particle spacing with fluid inertia was observed recently, little is known concerning the mechanism of particle self-assembly.
Samples for flow cytometers are suspended cells, which aligned with pinched flow and pass through optical sensing region. Focusing of particle using inertial microfluidic system can have accuracy better than ˜0.1 μm and operate without pinch flow thereby enabling parallelization. Sensing efficiency and throughput can be enhanced by uniform spacing because sensing signal in frequency domain will have narrow bandwidth, the particle stream will not have overlaps, and empty space can be reduced. There has been no techniques utilizing channel geometry to reduce hydrodynamic interaction.